1. Field of the Invention
The present invention relates to an element structure having a space therewithin and a method for producing the same.
2. Description of the Related Art
FIG. 5a shows a schematic sectional view of an acceleration sensor 40. The acceleration sensor 40 is a type of element structure having a space therewithin. FIG. 5b shows a schematic top view of an SOI (silicon-on-insulator) substrate 5 of the acceleration sensor 40. FIG. 5a is a fragmentary sectional view of the acceleration sensor 40 taken along line A-A in FIG. 5b. 
The acceleration sensor 40 includes a floating weight 2. A securing member 3 is disposed around the weight 2 with a space therebetween. The weight 2 is held and secured to the securing member 3 with a beam 4. The beam 4 has a piezoelectric resistor (not shown) whose electrical resistance varies according to the changes in stress. The beam 4 and the securing member 3 are provided with a wiring pattern (not shown) on the surface thereof. The wiring pattern is electrically connected to the piezoelectric resistor.
The weight 2, the securing member 3, and the beam 4 are integrated by etching a common SOI substrate 5. The SOI substrate 5 includes the following elements (not shown): a silicon support layer, a silicon oxide insulating layer and a silicon SOI layer in that order from below. A polyimide layer 6A is formed on the front surface of the SOI substrate 5. Also, a polyimide layer 6B is formed on the rear surface of the SOI substrate 5. These polyimide layers 6A and 6B have holes UA and UB, respectively, located in the regions opposing the weight 2 and the beam 4. A glass substrate 7A having a polyimide layer 8A is disposed over the polyimide layer 6A. The polyimide layer 6A is bonded to the polyimide layer 8A with an adhesive 9A. Also, a glass substrate 7B having a polyimide layer 8B is disposed under the polyimide layer 6B. The polyimide layer 6B is bonded to the polyimide layer 8B with an adhesive 9B. By bonding the polyimide layer 6A to the polyimide layer 8A with an adhesive, and by bonding the polyimide layer 6B to the polyimide 8B with an adhesive, the SOI substrate 5 and the glass substrates 7A and 7B are combined.
The bonded structure 10A of the polyimide layers 6A and 8A with the adhesive 9A has a recess defined by the hole UA of the polyimide layer 6A. The bonded structure 10B of the polyimide layers 6B and 8B with the adhesive 9B has a recess defined by the hole UB of the polyimide layer 6B. These recesses define a space between the polyimide layer 8A and the weight 2 and beam 4, and a space between the polyimide layer 8B and the weight 2. The weight 2 and the beam 4 are enclosed for movement in the Z direction shown in FIGS. 5a and 5b within the bonded composite of the SOI substrate 5 and the glass substrates 7A and 7B. The weight 2 and the beam 4 are protected from the external environment by an enclosure. The polyimide layers 8A and 8B limit the movement of the weight 2 and, consequently, prevent abnormal plastic deformation and breakage resulting from excessive movement of the weight 2.
In the acceleration sensor 40, when an acceleration in the Z direction arises, the beam 4 is deformed to move the weight 2 in the Z direction. The movement of the weight 2 and the deformation of the beam 4 in the Z direction are increased as the acceleration in the Z direction is increased, and thus depend on the magnitude of the acceleration in the Z direction. The deformation of the beam 4 can be measured by detecting a signal according to the electrical resistance of the piezoelectric resistor provided at the beam 4, and the magnitude of the acceleration in the Z direction can be determined from the deformation of the beam 4.
In the above acceleration sensor 40, the polyimide layers 6A and 8A are bonded with the adhesive 9A, and the polyimide layers 6B and 8B are bonded with the adhesive 9B. It is however very difficult to apply a desired amount of adhesive (9A and 9B) to the polyimide layers 6A and 6B (8A and 8B). In addition, the pressure placed on the polyimide layers 6A, 8A, 6B, and 8B is varied in the step of bonding the polyimide layers 6A and 8A or the polyimide layers 6B and 8B with an adhesive 9A or 9B. Consequently, the thicknesses of the adhesives 9A and 9B are varied. If the adhesives 9A and 9B have small thicknesses, the distances between the weight 2 and the polyimide layers 8A and 8B is reduced; and if the adhesives 9A and 9B have large thicknesses, the distances between the weight 2 and the polyimide layers 8A and 8B are increased. Hence, the variations in thicknesses of the adhesives 9A and 9B undesirably varies the distances between the weight 2 and the polyimide layers 8A and 8B. The distances between the weight 2 and the polyimide layers 8A and 8B regulate the movement of the weight 2 and are involved in the performance of acceleration detection and the durability. Accordingly, variations in distance between the weight 2 and the polyimide layers 8A and 8B negatively affect the performance of acceleration detection and the durability.
In addition, if the amount of the adhesives 9A and 9B applied to the polyimide layers 6A and 6B (8A and 8B) in a manufacturing process of the acceleration sensor 40 is small, the adhesions between the polyimide layers 6A and 8A and between the polyimide layers 6B and 8B are undesirably reduced. In contrast, if the amount of the adhesives 9A and 9B is excessively large, the excess of the adhesives 9A and 9B undesirably spreads to the space around the weight 2 from the boundaries between the polyimide layers 6A and 8A and between the polyimide layers 6B and 8B. Furthermore, since the adhesives 9A and 9B contain an organic solvent, voids are formed in the cured adhesives 9A and 9B by volatilizing the solvent. The voids reduce the adhesion between the polyimide layers 6A and 8A and between the polyimide layers 6B and 8B. Also, the amount of voids and the shape of the voids vary the thicknesses of the adhesives 9A and 9B and accordingly vary the distances between the weight 2 and the polyimide layers 8A and 8B.